16 THE president's ADDRESS. 



apparently as harmless as T. lewisi to rats. By the agency of 

 tsetse-flies, T. bruxii is taken up from its natural hosts, and 

 inoculated into domesticated animals of various kinds, in which it 

 flourishes, and to which it is extremely fatal. To the native 

 races of domestic animals it is less deadly, but imported animals, 

 new to the country, succumb rapidly. Thus the wild game acts 

 as a " reservoir," from which the infection of 7\ brucii spreads to 

 other animals. 



From these facts it is seen that T. brucii is most harmless to 

 those hosts in which it is a " natural " parasite, that is to say, 

 in which there is what may be termed a racial adaptation of the 

 host to the parasites, and which are constitutionally able to 

 withstand the disease-producing powers of the parasite. On the 

 other hand, it is most deadly to those races in which it is a new 

 parasite, and which possess no constitutional power of defence. 

 The state of affairs is perfectly comparable to that seen in such 

 a disease as measles, which is a comparatively harmless disease 

 in Europeans, but an extremely deadly disease amongst races in 

 which it is freshly introduced. What is the precise nature of 

 the mechanism of attack and defence is the fundamental problem 

 of the researches on immunity which are now being conducted 

 with such ardour and industry b}' pathologists all over the world, 

 and which cannot be discussed now. The facts of parasitism 

 indicate clearly, however, that when a race of hosts has been 

 long associated with a race of parasites, the hosts gradually 

 acquire constitutional powers of resistance to the parasite, which 

 the hosts do not possess when first brought into relations with 

 the parasite. Hence it may be said that a new parasite will 

 generally be a dangerous one, though it does not necessarily 

 follow that the converse is true, and that a dangerous parasite 

 is always a new one. 



Here it must be pointed out that there are two ways in which 

 a given species of parasite is " new " to a species of host. On 

 general gi-ounds we must suppose that a parasitic organism of 

 any kind is descended from ancestors which were not parasitic. 

 Hence there must have been a period in the evolution of any 

 parasite when its ancestors were first adapting themselves to 



